Method in the manufacture of chemomechanical pulps

ABSTRACT

A METHOD OF PRODUCING HIGH YIELD PULP BY IMPREGNATING WOOD CHIPS OR OTHER LIGNOCELLULOSE CONTAINING MATERIAL WITH A SOLUTION SELECTED FROM THE GROUP CONSISTING OF ALKALINE HYDROXIDES AND ALKALINE CARBONATES, AT A TEMPERATURE BELOW 100*C. THE IMPREGNATED CHIPS ARE COOKED IN A DIGESTER IN AN ATMOSPHERE OF SATURATED STEAM AT A TEMPERATURE EXCEEDING 100*C. AND UNDER CORRESPONDING SUPERATMOSPHERIC PRESSURE, WHILE SIMULTANEOUSLY INTRODUCING SULFUR DIOXIDE INTO THE DIGESTER FOR A PERIOD OF TIME RANGING FROM 1-20 MINUTES AND IN SUFFICIENT QUANTITY TO REACT WITH THE BASE CONTAINED IN THE WOOD CHIPS TO FORM AN ALKALI SULFITE COMPOUND. THE THUS COOKED OR DIGESTED WOOD CHIPS ARE THEN DEFIBRATED UNDER SUBSTANTIALLY THE SAME PRESSURE AND TEMPERATURE CONDITIONS PREVAILING IN THE COOKER. THE PREFERRED IMPREGNATION TEMPERATURE IS FROM 30-40*C. AND THE PREFERRED COOKING TEMPERATURE IS FROM 160-180*C.

United States Patent Int. Cl. nzlc 3/06, 3/26 US. Cl. 162-19 6 Claims ABSTRACT OF THE DISCLOSURE A method of producing high yield pulp by impregnating wood chips or other lignocellulose containing material with a solution selected from the group consisting of alkaline hydroxides and alkaline carbonates, at a temperature below 100 C. The impregnated chips are cooked in a digester in an atmosphere of saturated steam at a temperature exceeding 100 C. and under corresponding superatmospheric pressure, while simultaneously introducing sulfur dioxide into the digester for a period of time ranging from 1-20 minutes and in suificient quantity to react with the base contained in the wood chips to form an alkali sulfite compound. The thus cooked or digested wood chips are then defibrated under substantially the same pressure and temperature conditions prevailing in the cooker. The preferred impregnation temperature is from 30-40 C. and the preferred cooking temperature is from 160-180 C.

This application is a continuation-in-part of applicants copending application Ser. No. 369,001 filed May 20, 1964, now abandoned.

The present invention relates to the manufacture of chemomechanical pulp from lignocellulose containing materials.

Particularly, this invention relates to a method of manufacture of chemomechanical pulps involving heating and defi'brating same in. an atmosphere of vapor at elevated temperature and under corresponding pressure of im pregnated chips.

The term chips, in this specification, will be understood to designate all kinds of lignocellulose material subject to disintegration.

It is known to produce chemomechanical pulps in which the yield is as high as 80 to 85 percent, by impregnating lignocellulose containing material with sulfite solutions of various pH value, and thereafter heating the impregnated material in an atmosphere of saturated steam under pressure at temperatures exceeding 100 C., preferably within the range of 150 C. and 200 C., and subsequently subjecting the impregnated material to defibration.

In such process it is of importance that the material become thoroughly impregnated if the finished pulp is to have a satisfactory brightness and mechanical strength. The equipment usually employed for such process comprises a steam container, a compression apparatus for the steamed chips and a vessel containing the impregnating solution in which the compressed chips are allowed to expand and take up sulfite solution, as is disclosed, for example, in copending application Ser. No. 237,911 filed Nov. 15, 1962, by Arne Johan Arthur Asplund.

Various kinds of wood differ greatly with respect to capacity for taking up sulfite solutions. It is for example, considerably easier to impregnate birch wood than pine wood chips.

Experiments have now shown that it is possible to obtain a more rapid and effective impregnation of the chips with alkali sulfite or alkali bisulfite or mixtures thereof, by initially impregnating the pulp with dilute alkali hydroxide or alkali carbonate solutions and thereafter rapidly converting the base into sulfite of desired composition and pH value in the digester simultaneously with the cooking process. This method is carried out in such a manner that chips impregnated with alkali hydroxide or carbonate are sluiced into a pressure vessel in which a vapor pressure is maintained by direct introduction of steam. Simultaneously, the pressure vessel is supplied with S0 in state of a gas, liquid or solution, in such amount that the base is converted into sulfite or bisulfite compounds or mixtures thereof.

By initially impregnating the chips with an alkali solution it is possible to operate under the temperature conditions at which the digestion is usually carried out, viz. C. to 180 C., and thereabove. In such cases where a short time of digestion such as 1 to 20 minutes is maintained it is also possible to operate with an excess of S0 in the steam without any appreciable decomposition of the cellulose substance taking place, the defibrated pulp moreover being of notably high brightness. When a sodium hydroxide or sodium carbonate solution is used for the impregnating liquid, the greatest amount of base (as sodium oxide Na O) the chips will take up depends on the nature and quality of the lignocellulose containing material and the yield and the pulp quality desired. The largest quantity of Na O which may come into consideration appears to be 8 percent calculated on a basis of the dry substance of the chips. Normally the quantity of Na O will be in the range of 1 and 5 percent. The concentration of the alkali solution is determined by the quantity of base which it is intended, the chips should take up, by the water content of the chips and by the method of application thereof in the performance of the impregnation.

The temperature at which the impregnation with alkali is carried out must be low, for example, within the range of 30 C. to 40 C., and in any event below 100 C., in order to avoid discoloration of the chips. The reaction of the base with S0 is carried out directly in the digester under normally prevailing cooking conditions, namely in a vapor phase and at a temperature exceeding 100 C. and at corresponding superatmospheric pressure. The disintegration of the chips should take place immediately after the completion of the impregnation with alkali. In order to avoid non-desired discoloration, any storing for a longer period of the chips impregnated with the alkali must be avoided.

EXAMPLE Pine chips are treated with steam at atmospheric pressure for less than five minutes and impregnated with NaOH in proportion corresponding to about 1.5 percent Na 0 calculated on a basis of the weight of the dry substance of the chips. The steamed chips are compressed in a screw press, as disclosed, for example, in the copending patent application referred to above. Thereupon the chips are allowed to expand in a soltuion of NaOH having a concentration of 3.5 percent. The chips from which excess of free impregnating liquid has been removed are sluiced continuously into a pressure vessel, the interior of which is maintained at a vapor pressure of 8 atmospheres above normal, and at a temperature of about C. by direct introduction of steam. Simultaneously, free gaseous S0 is supplied to the pressure vessel in such quantity that an excess of free S0 is maintained in the steam. The chips are heated for 1 to 2 minutes in the acid steam atmosphere and immediately thereafter defibrated under the pressure and temperature conditions prevailing in the pressure vessel. The treated chips are removed by being passed through a sluice device into a zone of atmospheric pressure. Upon dilution with water to a concentration of percent the pulp was refined to a freeness of 290 CSF.

The yield of pulp reached 92 percent and the product showed the following quality values:

Brightness, percent GE 60 Breaking length, meters 3,800 Burst factor 14.7 Folding test, MIT 1 520 Tear factor (Elmendorf) 70 1 Load 300 grams.

With the method hereinbefore described of impregnating chips with sulfite solutions, a more rapid and effective impregnation is obtained than is obtainable by direct application of sulfite solutions. It is possible to make use of simpler material in the impregnating apparatus and no special apparatus for production of sulfite solutions is required thereby considerably reducing the costs for erecting the plant.

Whereas a specific embodiment of the invention has been described, it will be understood that this is for purposes of illustration, and it not in any way to be taken as a limitation on the scope of the invention as set forth in the subjoined claims.

What We claim is:

1. The method of continuously producing chemomechanical pulp from lignocellulose containing material, which comprises:

(a) impregnating the material with a dilute solution of a substance selected from the group consisting of alkali hydroxides and alkali carbonates at a temperature below 100 C. and in a quantity to produce a content of base in the material ranging from 1 to 8 percent of dry material;

(b) compressing said material and removing free impregnating solution from the material;

(0) cooking the impregnated material in an atmosphere of saturated steam at a temperature exceeding 100 C. and under corresponding pressure;

(d) reacting the base contained in said material by introducing sulfur dioxide simultaneously with the said cooking step in such quantity and for a period of time ranging from 1 to 20 minutes such as to produce alkali sulfite compounds in the material; and then (e) defibrating the thus treated lignocellulose containing material.

2. Method according to claim 1 in which the temperature during the impregnating step is maintained substantially between 30 C.40 C.

3. Method according to claim 1 in which the quantity of sulfur dioxide supplied during the cooking step is sufficient to create free sulfur dioxide in the atmosphere of steam.

4. Method according to claim 1 in which the temperature during the cooking step is substantially within the range of C. C. and at a corresponding superatmospheric pressure.

5. Method according to claim 1 in which the defibrating step is carried out under substantially the same temperature and pressure conditions as those prevailing during the cooking step.

6. Method according to claim 1 in which the concentration of the alkaline substance in dilute solution is substantially about 3.5%.

References Cited UNITED STATES PATENTS 1,849,334 3/1932 Richter 162-83 1,887,899 11/1932 Bradley et al. l6286 FOREIGN PATENTS 409,081 4/1934 Great Britain l6286 S. LEON BASHORE, Primary Examiner A. DANDREA, JR., Assistant Examiner U.S. Cl. X.R. l6256, 64, 86 

